A conventional magneto-optical recording medium is composed of a substrate in a disk form, and a magnetic layer with perpendicular magnetization provided on the substrate. The magnetic layer is made of, for example, rare earth-transition metal alloys. The recording and reproducing operations with respect to the magnetic layer of the magneto-optical recording medium is performed in the manner presented below.
Prior to the recording operation, first, in order to initialize the recording medium, the magnetizations are directed to a single specific direction (upward or downward) in accordance with a externally applied strong magnetic field, namely, an initializing magnetic field. Next, a laser beam is projected onto a recording area of the recording medium so as to raise temperature thereof above the vicinity of its Curie temperature or above the vicinity of its compensation temperature. As a result, coercive force (Hc) in the projected area becomes zero or nearly zero. In this state, an externally applied magnetic field having a direction opposite to that of the initializing magnetic field is applied, thereby reversing the magnetization direction. After the projection of the laser beam is stopped, the temperature of the recording medium drops to room temperature, and the reversed magnetization direction is fixed, thereby recording information thermomagnetically in accordance with respective directions of magnetization. When the reproducing operation is to be carried out, a linearly polarized laser beam is projected onto the recording medium, and the recorded information is optically read out utilizing an effect, such as magnetic Kerr effect or magnetic Faraday effect, that the polarization plane of reflected or transmitted light rotates according to the magnetization direction.
The magneto-optical recording medium has been viewed with interest as a rewritable high density and large capacity memory device. As a method for rewriting information on the magneto-optical recording medium, a method of overwriting by the light intensity modulation has been proposed. This light modulation overwriting method is for use with a magneto-optical recording medium which has a recording layer composed of two reciprocally exchange-coupled magnetic films. According to the method, an initializing magnetic field (Hi) and a recording magnetic field (Hw) are adopted respectively when initializing and recording, and information is rewritten by modulating the light intensity of the laser beam projected on the recording medium.
Another magneto-optical recording medium has been proposed by Japanese Examined Patent Publication No. 5-22303/1993, for use with the foregoing method of overwriting. The proposed magneto-optical recording medium has a triple-layered recording layer on a substrate 21 (see FIG. 7), so that the initializing magnetic field (Hi) is reduced and the stability of recording bits is enhanced.
Thus, the magneto-optical recording medium has a first magnetic layer 22, a second magnetic layer 23, and a third magnetic layer 24. The first magnetic layer 22 is a magnetic thin film with a perpendicular magnetization, which has a low Curie temperature and great coercive force. The third magnetic layer 24 is a magnetic thin film with a perpendicular magnetization, which has a relatively high Curie temperature and relatively small coercive force in comparison with the first magnetic layer 22. The second magnetic layer 23, provided between the first and third magnetic layers 22 and 24, has an in-plane magnetization at room temperature while a perpendicular magnetization as temperature rises.
The initializing magnetic field (Hi) 27a is set smaller than the coercive force of the first magnetic layer 22 at room temperature, while greater than the coercive force of the third magnetic layer 24 at room temperature. Therefore, the direction of the magnetization of the first magnetic layer 22 is not reversed by the initializing magnetic field (Hi) 27a at room temperature.
Since the second magnetic layer 23 is arranged so as to have an in-plane magnetization at room temperature, such an arrangement prevents, at room temperature, the magnetic coupling of the first magnetic layer 22 and the third magnetic layer 24 based on the exchange-coupling force.
The following description will explain the procedure of an overwriting operation with respect to such a magneto-optical recording medium. First, for the initialization, the initializing magnetic field (Hi) 27a is applied to the magneto-optical recording medium, thereby directing the magnetization in only the third magnetic layer 24 in one direction, for example, downward as shown in FIG. 7.
Then, a recording operation is performed by projecting a laser beam 25 on the magnetic layers 22-24 while applying on the layers a recording magnetic field (Hw) 28 whose direction of the magnetic field is directed in the direction opposite to that of the initializing magnetic field (Hi) 27a, that is, upward in this case. The laser beam 25, converged by an objective lens 26 onto the first magnetic layer 22, is modulated so as to have light intensity of its power between high power and low power, thereby varying the direction of the magnetization of the first magnetic layer 22 in accordance with the modulated light intensity. Information is recorded in accordance with the direction of the magnetization, which is thus varied.
The power of the laser beam 25 is arranged so that the high power laser beam 25 raises the temperature of the recording medium to T3, which is in the vicinity of the Curie temperature of the third magnetic layer 24, whereas the low power laser beam 25 raises the temperature of the recording medium to T2, which is in the vicinity of the Curie temperature of the first magnetic layer 22.
Therefore, when the high power laser beam 25 is projected while the recording magnetic field (Hw) 28a is applied to the third magnetic layer 24, the direction of the magnetization of the third magnetic layer 24 is reversed upward. Then, in the process of cooling off, by the exchange-coupling force exerted on an interface of the magnetic layers, the magnetization direction of the third magnetic layer 24 is copied to the second magnetic layer 23, which now has a perpendicular magnetization due to a rise in temperature, and then to the first magnetic layer 22. Thus, the direction of the magnetization of the first magnetic layer 22 is directed upward.
On the other hand, when the low power laser beam 25 is projected while the recording magnetic field (Hw) 28a is applied to the third magnetic layer 24, the direction of the magnetization of the third magnetic layer 24 remains unchanged, because the coercive force of the third magnetic layer 24 is greater than the recording magnetic field (Hw) 28a. As mentioned above, the direction of the magnetization of the third magnetic layer 24 is copied to the first magnetic layer 22 through the intermediary of the second magnetic layer 23, by the exchange-coupling force exerted on the interface of the magnetic layers in the process of cooling off. Accordingly, the direction of the magnetization of the first magnetic layer 22 is directed downward.
Note that the power of the laser beam in reproducing is set considerably smaller than the low power of the laser beam in recording. Therefore, the second magnetic layer 23 is arranged so that its in-plane magnetization is unaffected by such a laser beam of reproducing-use power. In other words, the second magnetic layer 23 prevents the direction of the magnetization of the third magnetic layer 24 from being copied to the first magnetic layer 22 by means of the exchange-coupling force.
As has been mentioned so far, with such an arrangement, the initializing magnetic field (Hi) 27a is set greater than the coercive force of the third magnetic layer 24 at room temperature, while the recording magnetic field (Hw) 28a is set in the vicinity of the midpoint between the respective coercive forces which the third magnetic layer 24 has at the respective temperatures when the high power laser beam 25 is projected and when the low power laser beam 25 is projected. Therefore, the initializing magnetic field (Hi) 27a is set considerably greater than the recording magnetic field (Hw) 28a. In addition, the initializing magnetic field (Hi) 27a is set to have a magnetization direction opposite to that of the recording magnetic field (Hw) 28a.
The above-mentioned conventional arrangement, however, has presented a problem that a recording device for recording information on a magneto-optical recording medium tends to be bulky, especially when the magnetic field generating units 27 and 28, for respectively generating the initializing field (Hi) 27a and the recording magnetic field (Hw) 28a are separately provided.
On the other hand, when the magnetic field generation units 27 and 28 are integrally provided, the conventional arrangement presents another problem that the control is required for reversing the direction of the magnetization of the recording magnetic field (Hw) 28a with respect to the initializing magnetic field (Hi) 27a. In addition, since the initializing magnetic field (Hi) 27a is greater than the recording magnetic field (Hw) 28a, the conventional arrangement in this case also presents the problem that such a unit for generating the magnetic fields tends to be bulky.